Sampling orifice

Portable or fixed multipoint colorimetric detectors are available which rely on paper tape impregnated with reagent. A cassette of the treated paper is driven electrically at constant speed over a sampling orifice and the stain intensity measured by an internal reflectometer to provide direct readout of concentration. Such instruments are available for a range of chemicals including those in Table 9.7. 

To provide sufficient time for the droplet evolution, one can increase the distance between the ES capillary and the sampling orifice or sampling capillary. However, this reduces greatly the sampling efficiency since the charged droplet density and... 

Fig. 11.15. Gas chromatography interfaces (jet separator, top membrane separator, bottom). In the jet separator, momentum of the heavier analyte molecules causes them to be sampled preferentially by the sampling orifice with respect to the helium carrier gas molecules (which diffuse away at a much higher rate). In the membrane separator, the analyte molecules are more soluble in the silicone membrane material leading to preferential permeability. Helium does not permeate the membrane with the same efficiency and is vented away.

The use of the volatile salt CH3CO2NH4 (1 mM) for controlling adduct formation showed similar improvements to the spectra (data not shown), whilst its relative volatility minimised the negative aspects of salt addition in ESI-MS (i.e. detector saturation and precipitation on the sampling orifice) [29],... 

Fig. 12.10. Micromass z-spray with MUX technology. Eight ESI sprayers plus one reference sprayer are adjusted around the sampling orifice. However, only one sprayer has access to the orifice at a time, the others are blinded by a rotating aperture. By courtesy of Waters Corporation, MS Technologies, Manchester, UK.

Neutral species are best sampled using molecular beam methods in which the neutral beam is modulated between the sampling orifice and the ionization chamber of the mass spectrometer and only the modulated component of the mass spectrometer output is recorded. This approach enables all neutral species, including radicals to be detected with a comparable sensitivity. If modulation techniques are not used, the sensitivity for detecting condensible or reactive species is much less than for non-condensible, non-reactive neutral molecules because of the much larger effective pumping speed for the former in the mass spectrometer chamber. However, the ease of installation of non-line-of-sight non-modulated... 

The skimmer cone is another metal cone, the tip of which has an orifice approximately 0.7 mm in diameter, that protrudes into the zone of silence, and is axially in-line with the sampling orifice as shown in Fig. 5.3. The ions from the zone of silence pass through the orifice in the skimmer cone, into a second intermediate vacuum chamber held at < 10 atm, as an ion beam The ion beam can then be focused by means of a series of ion lenses, which deflect the ions along a narrow path and focus them on to the entrance to the mass analyser. 

Flow Toward the Sampling Orifice. The assumption that flow toward the sampling orifice is unlikely to significantly distort the reactor flow or to be a factor in reactant sampling can be justified with compressible flow arguments. For isentropic flow of an ideal gas the temperature and pressure are related to the Mach number by Eqs. 7 and 8 [40] ... 

Gas flow through the sampling orifice is of interest since its volume rate of flow determines the size of vacuum pumps necessary for the mass spectrometer. In addition, orifice flow behaves as a first-order removal process that can lead to erroneous kinetics if it is too large. The perturbation of flow in the reactor has been discussed in Section IV.B. 

Droplets in the aerosol shrink as the solvent evaporates, thereby concentrating the charged sample ions. When the electrostatic repulsion among the charged sample ions reaches a critical point, the droplet undergoes a so-called Coulombic explosion, which releases the sample ions into the vapor phase. The vapor phase ions are focused with a number of sampling orifices into the mass analyzer. 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

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